Micro-electro-mechanical systems (MEMS) are gaining increasing attention because of their potential advantages in miniaturization, performance, reliability, environmental endurance, low cost, and mass production capability. Various applications have been developed from MEMS devices. Microphones, bio devices such as micro flow pumps, and micro inkjet heads are among the most commonly seen applications of MEMS devices.
The formation of micro flow pumps and micro inkjet heads often involves forming channels (tunnels) for conducting fluid into or out of the corresponding MEMS devices. FIGS. 1 through 3 illustrate the cross-sectional views of intermediate stages in the formation of a channel. Referring to FIG. 1, MEMS device 4 is first formed on a front surface of substrate 2. In FIG. 2, channel 6 is formed from the backside of substrate 2, for example, by forming photo resist 8 on the back of substrate 2, patterning photo resist 8, and etching substrate 2. MEMS device 4 is exposed through channel 6. Next, as shown in FIG. 3, photo resist 8 is removed. The resulting MEMS device 4 is connected to channel 6.
In the process as shown in FIGS. 1 through 3, the etching of substrate 2 needs to stop when the etchant reaches MEMS device 4. It is thus required that the etching characteristic of MEMS device 4 be different from that of substrate 2, so that the etching selectivity may be high enough. This limits the usable materials and the structures of MEMS device 4, and puts a high accuracy requirement on the etching of substrate 2. Further, etching substrate 2 requires accurate alignment. If channel 6 is not straight, or the position of channel 6 offsets from the desirable position, MEMS device 4 may not be able to function correctly. For example, the fluid in channel 6 may not effectively flow into MEMS device 4. New methods for forming the channels of MEMS devices are thus needed.